Ion-Assisted Aquation

This general area has recently been reviewed, however, the mechanism of Hg assisted chloride release reactions from [CrCUNs)] is now believed to be /a rather than D or The spontaneous acid hydrolysis of [Co(ox)2(hist)]  

The final product, [Cr(OH2)4(hist)], has been used in thiocyanate anation studies (Section 6.3.2) and the relative resistance of this complex to Cr—N bond rupture illustrates the increased stability of six-membered N-donor chelate rings when compared to five-membered analogs. 

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The interaction of silver ions with [Co(NH3)5NCS]2+ was first reported in 1899 to give a stable silver adduct [Co(NH3)5NCSAg]3+.147b More recently similar adducts have been prepared as part of studies on the silver ion-assisted aquations of this and other related complexes and UV and IR data were reported.147 ... 

More closely related to kinetics and mechanism are three reviews from Danish chemists, one on the state of the art with respect to Cr(III) substitution reactions and the other two ° survey /t-diol dinuclear Cr(III) systems. Chromium chemistry is also discussed in a general review on metal-ion-assisted aquation reactions " and in a review on the effect of pressure on reaction rates. In the following sections, units for and AH are in kJ mol A5 in J K mol A V in cm mol visible absorption spectral maxima (A ax) in nm, and molar extinction coefficients (e) in cm Unless otherwise stated, all rate constants are reported at 25.0 (298.2 K). 

Because of its very fast rate of aquation in acidic solutions, only a minute amount of the pentacyanochromium(III) complex (< 1 %) exists in the hydrolysis solutions and is difficult to isolate from such solutions. The ion can be isolated in small amounts, however, if the acid-assisted aquation reaction is quenched at an early stage in the reaction and if most of the unreacted Cr(CN) " is removed by precipitation prior to the ion exchange separation. The procedure is as follows. ... 

Another way in which added metal ions might affect aquation rates of the [IrCle] anion is by direct catalysis of the Co(III)-Cl" plus mercury(II) type (cf. Section 5.7.3). Such metal ion assistance could be involved for Ga " as it is... 

The metal-ion-catalyzed dissociation of oxalato complexes of the type [Cr(ox)(OH2)4]", [Cr(ox)2(OH2)2], and [Cr(ox)3] have been investi-gated/ The pseudo-first-order rate constant (fcobs) is given by fcobs = k + /cm[M ], and varies for different metal ions (M ) in the order Fe " Cu > Ni " > Zn > Co " > Mn. This order reflects the ease of binding of to a chromium(III)-coordinated oxalato group. Protons also assist oxalate dissociation, with k between the values of k obtained for Co " and Mn. The rate data are summarized in Table 6.4. A very similar study of the metal-ion-catalyzed aquation of [Cr(mal)3] ion (mal = malonate ion) has appeared. The data are presented in Table 6.5. 

Activation parameters for the path corresponding to the A i[Fe +]term are — 24.2 0.5 kcal mol and IsS — 8.9 1.6caldeg mol The iron(m) assists the oxalate group to leave by co-ordinating to it, possibly in a unidentate-oxalate intermediate rather than in the starting complex itself. Several examples of metal-ion-catalysed aquation of chromium(ra) complexes are included in the general rate constant against stability constant correlation discussed in the section on catalysed aquation of cobalt(m) complexes. 

Decarboxylation is assisted by micellized pyridinium ion Aquation is accelerated by dodecyl-pyrazinium chloride... 

Metal ions can assist in the dissociation (hydrolysis) of complexes containing multidentate ligands. The metal ion may not necessarily complex with the detached ligand, for example, in the metal-assisted acid-catalyzed aquation of Cr(C204)3. Ref. 112. Usually however the metal ion removes and complexes the ligand as in... 

Aquation rate constants [Co(NH,), X]", 34 165-166 ion pairing effects, 34 173 metal-assisted reactions, 34 168-169 pentaamminecobalt(III) complexes, 34 149 pentaamminemetal(III) complexes, 34 152-153... 

Rates of the acid, Hg ion-induced, and base hydrolyses of c/y-[Co(tmd)t-(RNH2)C1] + ions (R = H, Me, Et, Pr , Bu , Bu or Ph) are ca. 10 faster than for the analogous [Co(en)2(RNH2)Cl] + ions. This arises primarily from a reduction in the activation energy by ca. 2.5 kcal mol . A comparison of the rate data for acid, Hg -assisted, and base hydrolysis reactions is given in Table 6. cis-trans isomerization was not observed in the aquation reactions. 

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